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Secretion of growth hormone (GH) exhibits marked sexual dimorphism in the rat. To examine the underlying mechanism that involves hypothalamic GH-releasing factor (GRF), we determined pituitary GRF receptor mRNA levels in male and female rats and compared their in vitro abilities to release GRF, an endogenous ligand for GRF receptor. Female rats expressed GRF receptor mRNA at a level of only 15% (P < 0.001) of that of male rats. Female rats also showed a 33% lower (P < 0.01) ability to release GRF than male rats. These results indicate that the GRF secretion and action system of female rats is characterized by the combined reduction in GRF receptor expression and GRF-releasing capacity compared with that of male rats. This could explain the in vivo finding that spontaneous, GRF-triggered GH pulses are of much lower amplitude in the female than in the male rat.
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